This update allows use of bower.json main value in buildtools that rely on
the appropriate files being defined in this main array. It also points to
the unminified, but compiled dist versions as per bower spec:
https://github.com/bower/bower.json-spec#main
* Changes main js file to use unminified version
* Adds the gridstack.css file to the main files array
* Moves compiled CSS into `dist` folder (upstream build process needs update)
* Duplicate `src/gridstack.js` into `dist/gridstack.js` (upstream build process needs update)
* Update demo html to point to `dist` instead of `src`
